Cracking open those tail light lenses is a mutha! the easiest way i have found is to buy a few feet of Red, Amber, and White LED strips and just get creative with how you place them.
i tried attaching some pics of how i had the red LED's on my taillights.( sorry so blury) I would have opened them up and placed them inside, but this is also my girlfriends car and i wanted EVERYTHING i installed on this car to be 100% removable with no signs of any of it when we go to trade it in or sell it.
but if you are dead set on opening up either the tail light lenses or head light lenses, easiest way i have found is to put them in the oven set on 300. do them one assembly at a time, and only keep them in for about 6 - 8 minutes a piece. then CAREFULLY PRY THEM APPART. You can also use a heat gun to melt the adheasive and pry the lense from the assembly but i have found the oven method to be the easiest. just dont leave them in there for too long.